The present invention relates to a system for regulating the oil and fuel temperatures of an aircraft turbojet engine by controlling the heat exchange between the oil and the fuel of the engine. More specifically, the invention concerns a temperature regulating system for a turbojet engine fitted with an alternator which is mechanically driven at constant speed through a power transmission driven by a rotating part of the turbojet engine.
Today's turbojet engines typically comprise two separate oil circuits, namely a first closed oil circuit to circulate and cool lubricating oil which lubricates the internal turbojet engine structures, such as bearings, and a second closed oil circuit to circulate and cool oil that lubricates the alternator drive means. Fuel flow feeding the fuel injectors of the turbojet engine is supplied from a fuel circuit drawing fresh fuel from the fuel tanks, typically located in the wings of the aircraft. To facilitate combustion, the fuel is heated in the fuel circuit to an elevated temperature having an upper limit which must not be exceeded in order to preclude partial fuel vaporization, or decomposition, which would cause engine fuel pumps and fuel regulators to malfunction.
The amount of fuel flowing into the turbojet engine depends upon the demands placed upon the engine by the command pilot. The oil flowing in the first closed oil circuit depends upon the rotational speed of the turbojet engine and the heat carried by this oil depends upon the turbojet engine load. Because the alternator is driven at a constant speed, the oil flow through the second oil circuit is substantially constant and the heat carried by this oil depends upon the aircraft power drain and on the heat dissipated by the integrated mechanical transmission connecting the alternator to the driving part of the turbojet engine. Such mechanical transmissions dissipate large quantities of heat even when the moving parts are rotating slowly. The thermal energy dissipated in the second oil circuit is between 5-15% of that dissipated in the first oil circuit during full power engine operation and may rise to 50% under engine idling conditions.
Present research into the designs of high bypass ratio fan jet engines include the use of rotational speed reducers to reduce the rotational speed of the fan driven by the turbojet engine. This research applies also to turboprop engines regarding the mechanical linkage between the engine and the propeller. Such speed reducers transmit high power and, accordingly, the heating of the speed reducer components assumes great design significance. Effective cooling of the oil in the first closed oil circuit must be achieved to maintain permissible engine temperature levels. In a speed reducer equipped engine, the thermal power to be dissipated is approximately twice as large as that to be dissipated in a conventional turbojet engine. This heat is now removed by using a high oil flow through the first closed circuit.
It is known to cool the oil passing through the first oil circuit by using an oil/fuel heat exchanger in which the heat from the oil is transferred to the fuel which is then supplied to the turbojet engine. The fuel temperature may not exceed 150.degree. C. in order to avoid the previously mentioned component malfunctions. Because of the high efficiency of turbojet engines with speed reducers and high bypass ratios, neither the fuel flow into the engine, nor the lubricating oil passing through the first closed oil circuit is sufficient to remove the heat generated in the alternator power transmission.
It has been proposed to remove the heat generated by the alternator power transmission using a special heat exchange fuel circuit and returning the heated fuel to the tanks in the aircraft wing with the wing surface assuming the function of a fuel/air heat exchanger. This suggestion does not solve the problem when the aircraft is operated in hot climates in which the fuel in the wings will overheat if the aircraft spends too much time on the ground awaiting takeoff when the engines are running. In such cases, it is necessary to stop the engines, drain the overheated fuel and replace it with fresh fuel. Conversely, in a cold operating environments, starting the engines fitted with speed reducers is also a problem because the oil may be less viscous and, under extreme conditions, congealed.
The state of the art is illustrated by U.S. Pat. Nos. 4,151,710; 4,741,152; 5,121,598; and 5,253,470. U.S. Pat. No. 4,151,710 describes an oil cooling system for turbojet engines in which the lubrication oil for the alternator drive transmission is cooled by a second air/oil heat exchanger connected in series with the previously mentioned oil/fuel heat exchanger and located downstream in the direction of fuel flow through the heat exchanger. In aircraft descent phases, the fuel which was heated by its recirculation, is cooled by the oil of the second closed oil circuit in the second heat exchanger, necessitating the use of an air/oil heat exchanger in the second closed oil circuit.
U.S. Pat. No. 4,741,152 utilizes two lubrication circuits, each comprising an air/oil and an oil/fuel heat exchanger. The oil/fuel heat exchanger of the cooling circuit for the alternator drive transmission is located upstream of the oil/fuel heat exchanger in the fuel circuit.
The systems noted in the aforementioned U.S. patents do not allow heating the oil of the first closed oil circuit by means of the oil in the second closed oil circuit when the turbojet engine is started in cold operating conditions.
U.S. Pat. No. 5,121,598 discloses cooling equipment for the lubrication oil of the alternator drive transmission which comprises an oil/fuel heat exchanger in an oil return circuit which returns the fuel to the wing tanks. U.S. Pat. No. 5,253,470 discloses an oil heating system using the heat generated by the engine starter.